The utilization of nuclear magnetic resonance (hereinafter referred to as "NMR") to create a gyroscope is disclosed in U.S. Letters Pat. No. 4,157,495 which issued June 5, 1979 and which is assigned to the same assignee as the present invention.
The gyroscope disclosed therein operates on the principle of sensing inertial angular rotation rate or angular displacement about a sensitive axis of the device as a shift in the Larmor precession frequency or phase, respectively, of one or more isotopes that possess nuclear magnetic moments.
The gyroscope is composed of an angular rotation sensor and associated electronics. The principal elements of the sensor are a light source, an NMR cell, a photodetector, a set of magnetic shields and a set of magnetic field coils. The principal elements of the electronics are signal processing circuits for extracting the Larmor precession frequency and phase information as well as circuits for generating and controlling various magnetic fields, both steady and varying sinusoidally with time, that are necessary for the proper operation of the device.
The NMR cell is mounted within a set of magnetic shields in order to attenuate external magnetic fields to acceptable low levels. Magnetic field coils are used to apply uniform magnetic fields to the NMR cell. Both a steady field and an AC carrier field are applied along the sensitive axis of the device and an AC feedback field is applied along one of the transverse axes. The DC magnetic fields along both transverse axes are controlled to be substantially zero. The NMR cell contains a single alkali metal vapor, such a rubidium, together with two isotopes of one or more noble gases, such as krypton or xenon. One or more buffer gases such as helium or nitrogen may also be contained in the cell.
The NMR cell is illuminated by a beam of circularly polarized light that originates from a source such as a rubidium lamp and which passes through the cell at an angle with respect to the steady magnetic field. Absorption of some of this light causes the atomic magnetic moments of the rubidium atoms to be partly aligned in the direction of the steady magnetic field. This alignment is partly transferred to the nuclear magnetic moments of the noble gases, and these moments are caused to precess about the direction of the steady magnetic filed, which in turn creates magnetic fields that rotate at the respective Larmor precession frequencies of the two noble gases. These rotating fields modulate the precessional motions of the rubidium magnetic moments, which in turn produce corresponding modulations of the transmitted light, thereby making it possible to optically detect the Larmor precession frequencies of the two noble gases.
The modulations of the light intensity are converted into electrical signals by a photodetector, and these signals are then electronically demodulated and filtered to provide signals at the Larmor precession frequencies of the two noble gases. The difference between the two precession frequencies is used to accurately control the steady magnetic field so that it is constant. One of the noble gas precession frequencies is subtracted from a precision reference frequency. The resulting difference frequency is a measure of the angular rotation rate of the gyroscope. The magnitude of an individual nuclear magnetic moment is extremely small and the natural equilibrium condition is one in which a nearly random orientation of moments exists in an ensemble of atoms. Techniques must be used to orient a significant fraction of these magnetic moments in a single direction so that a macroscopic magnetic moment, and consequently a measurable signal, will be produced.
The aligned magnetic moments of the single alkali metal system and of both noble gas systems of atoms are subject to relaxation mechanisms which cause their alignments to decay with time towards their natural equilibrium condition of random orientation. Each system of moments is characterized by a relaxation time constant which depends on the kinds and quantities of all other constituents and upon the total environment in the NMR cell. The steady state fractional alignment of each system of moments is a function of both the pumping rate and the relaxation time for the system, with larger fractional alignments, hence larger signal amplitudes, being achieved when the relaxation times are also long.
A number of prior art techniques exist to achieve longer relaxation times. In one of the techniques, a suitable amount of a buffer gas such as helium or nitrogen is also contained in the cell in order to reduce the relaxation effects due to interactions of the magnetic moments with the walls of the cell. In another technique, particular isotopes of particular noble gases are chosen as the nuclear magnetic moment gases specifically for their long relaxation times. However, a problem still exists since certain, otherwise desirable magnetic moment gases have relaxation times too short to provide a practical device.
Two co-pending patent applications which address this problem and which are assigned to the same assignee as the present invention include Ser. No. 307,995 filed Oct. 2, 1981, for an Improved Magnetic Resonance Cell And Method For Its Fabrication by T. M. Kwon and W. P. Debley, and Ser. No. 307,996 filed Oct. 2, 1981 for An Improved Magnetic Resonance Cell by T. M. Kwon and C. H. Volk.
In addition to the desire to lengthen the relaxation times of the NMR cell, it is also desirable to improve the temperature sensitivity of the cell. It is known that an NMR cell will undergo a drift measured in degrees per hour. That is, by placing an NMR cell within a gyroscope upon a fixed platform and plotting the apparent angular rotational rate of the gyroscope in degrees, one will find that the output of the gyroscope produces an indication of rotation even though the gyroscope has been fixed. This apparent rotation is referred to as the gyro bias. It has been found that the gyro bias is influenced by temperature.